Oscillations, Waves, and Interactions - GWDG

32 H. W. Strube
3.5.1 Glottis pulse parameters by inverse filtering
For describing the glottal oscillation, it is hardly feasible to estimate the many mechanical
parameters of the vocal-fold system, but characteristic quantities can be
used as occurring in parametric models of the pulse shape of the glottal volume velocity,
for instance, the well-known LF-model [29]. Such a model can be fitted to a
measured pulse shape. For the measurement to be feasible during normal speaking,
only inverse-filtering techniques are applicable. That is, the filtering by the vocal
tract is computationally undone in order to obtain the glottal volume velocity. This
leads to the circular problem that the vocal-tract transfer function cannot be estimated
exactly enough without knowledge of the input signal. Thus, a simultaneous
estimation of the transfer function and the glottal pulse (as LF-model) was carried
out iteratively with a multidimensional optimization method, see Fig. 4.
The transfer function was computed in a pitch-related way with a modified DAP algorithm
[32] and the Itakura-Saito error occurring therein was also used as optimality
criterion for the total iteration. The method was verified with synthetic speech, where
beside the LF parameters also derived quantities such as Open-Quotient, Closed-
Quotient, Speed-Quotient, Parabolic Spectral Parameter were compared with the
given data. For natural speech, the energies of 200-ms intervals and the electroglottographically
measured open-quotient were employed for comparison with the model.
A detailed description can be found in Refs. [30,31].
Choice of
parameter set
Winner
set
Multi−dimensional
optimizer
0.4
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
Glottis model
Spectrum
without glottis
Error
Speech signal
2.1 2.105 2.11 2.115 2.12 2.125
Time [s]
Inverse filtering,
DAP
Inverse filtered
signal
Figure 4. Top: Scheme of estimating glottal pulse and transfer function, cf. [30,31]. Bottom:
Example of estimated glottal flow derivative and fitted LF-model (gray, smooth).